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IGRT One requirement for the precise application of high radiation dosages to pre-determined target areas is being able to reliably position the patient in exactly the same way for each session. Spatial irradiation errors can result in a reduction of the dosage within the target volume as well as an increase in the dosage to critical healthy tissue such as the rectum and bladder in the case of a prostate treatment. Moreover, position variability necessitates the addition of safety margins to the irradiation target area. These safety margins include normal healthy tissue that is then also exposed to the prescribed (high) radiation dosage. In addition to the possibility of mis-positioning the patient, which can be minimized by appropriate fixation techniques, the variability in the position of the prostate and seminal vesicles within the pelvis - dependent on bladder and rectum content - must also be taken into account. Numerous methods to reduce position variability have been developed. In contrast to the more or less invasive methods such as the daily insertion of specially sized rectal balloons or the permanent implantation of gold fiducial markers which can be detected by x-ray, non-invasive methods are becoming increasingly available. Two of the non-invasive methods offered by our department are absed on: - Computertomographic Method - Ultrasound Method Both techniques have the potential to markedly increase the effectiveness of radiotherapy through more exact dose administration and minimization of undesired radiation dosage to healthy tissue by decreasing the necessary safety margins.   Computertomographic based Method In order to carry out a computer tomography (CT) scan in the treatment room when the patient is in position, a CT attachment called a Cone-Beam CT is placed on the radiation unit. A cone-beam CT is available in Mannheim (ill. 1).   Ill.1 Cone-beam CT Telecast "Cone beam CT Mannheim " With the imaging system directly mounted on the radiation unit, a 3D image of the target region inside the patient can be generated inside of a minute with a single gantry rotation. This image shows the actual state inside the patient and can be superimposed on images of the nominal state (for example from the radiation dosage plan) for comparison and used to determine how the patient should be repositioned if necessary.   Ill.2 Superimposed image combining Cone-beam and treatmentplanning CT   Ultrasound based Method A further method, already in widespread use in the United States, which has been implemented for the first time in Europe in our clinic in Mannheim, is the daily localization and positioning of the prostate with stereotactic Ultrasound.   Ill.3 Ultrasoundsystem (BAT) In this method, the position and shape of the target and at-risk anatomic structures (for example, the prostate, seminal vesicles, rectum, and bladder) are transferred from the radiation plan, where they are defined on the basis of CT and/or MR scans, onto a specially modified Ultrasound device (BATTM, Nomos Corp, Sewickley, Pennsylvania, see Ill. 2). The device's Ultrasound head and with it the image layer are calibrated to the beam geometry of the radiation unit. In just a few minutes (under five) the nominal position of the target organ, for example the prostate, can be compared daily with the actual position. When necessary, the position of the patient can be immediately corrected. This means making a virtual shift until the outlines of the organ structures from the ultrasound image match the outlines from the irradiation planning program perfectly.   Ill.4 Ultrasound images (in two layers) bevor correction Ill.5 Ultrasound images (in two layers) after correction The BAT Ultrasound device shows technicians how far the treatment table - and with it the patient - needs to be shifted along the three primary axes (right/left, up/down, and in the direction of the head/feet) - in order to position the prostate exactly in the irradiation field. Ideally, repeatedly measuring and checking the position of the target organ allows the patient to be perfectly positioned. When properly applied, this technique allows a very high degree of accuracy with mean positioning errors on the order of only 2 to 4 millimeters and only marginal variation between different operators.   Verantwortlich:  V.Steil letzte Änderungen:  14.09.2007 Webmaster